Field of the Invention
Embodiments of the invention relate to an organic light emitting diode (OLED) and more particularly to a delayed fluorescence compound having excellent emitting efficiency and an OLED and a display device using the delayed fluorescence compound.
Discussion of the Related Art
The requirements of large-size display devices have led to developments in flat panel display devices as an image displaying device. Among the flat panel display devices, the OLED has rapidly developed.
In the OLED, when the electron from a cathode, which serves as an electron-injecting electrode, and a hole from an anode, which serves as a hole-injecting electrode, are injected into an emitting material layer, the electron and the hole are combined and become extinct such that the light is emitted from the OLED. A flexible substrate, for example, a plastic substrate, can be used as a base substrate for the OLED, and the OLED has excellent characteristics of driving voltage, power consumption, and color purity.
The OLED includes a first electrode as an anode on a substrate, a second electrode as a cathode facing the first electrode, and an organic emitting layer therebetween.
To improve the emitting efficiency, the organic emitting layer may include a hole injection layer (HIL), a hole transporting layer (HTL), an emitting material layer (EML), an electron transporting layer (HTL), and an electron injection layer (EIL) sequentially stacked on the first electrode.
The hole is transferred into the EML from the first electrode through the HIL and the HTL, and the electron is transferred into the EML from the second electrode through the EIL and the ETL.
The electron and the hole are combined in the EML to generated excitons, and the excitons are transited from an excited state to a ground state such that light is emitted.
The external quantum efficiency of the emitting material for the EML can be expressed by:ηext=ηint×Γ×Φ×ηout-coupling 
In the above equation, “int” is the internal quantum efficiency, “r” is the charge balance factor, “Φ” is the radiative quantum efficiency, and “ηout-coupling” is the out-coupling efficiency.
The charge balance factor “Γ” means a balance between the hole and the electron when generating the exciton. Generally, assuming 1:1 matching of the hole and the electrode, the charge balance factor has a value of “1”. The radiative quantum efficiency “Φ” is a value regarding an effective emitting efficiency of the emitting material. In the host-dopant system, the radiative quantum efficiency depends on a fluorescent quantum efficiency of the dopant.
The internal quantum efficiency “ηint” is a ratio of the excitons generating the light to the excitons generated by the combination of holes and electrons. In the fluorescent compound, a maximum value of the internal quantum efficiency is 0.25. When the hole and the electron are combined to generate the exciton, a ratio of singlet excitons to triplet excitons is 1:3 according to the spin structure. However, in the fluorescent compound, only the singlet excitons, excluding the triplet excitons, are engaged in the emission.
The out-coupling efficiency “ηout-coupling” is a ratio of the light emitted from the display device to the light emitted from the EML. When the isotropic compounds are deposited in a thermal evaporation method to form a thin film, the emitting materials are randomly oriented. In this instance, the out-coupling efficiency of the display device may be assumed to be 0.2.
Accordingly, the maximum emitting efficiency of the OLED, including the fluorescent compound as the emitting material, is less than approximately 5%.
To overcome the disadvantage of the emitting efficiency of the fluorescent compound, the phosphorescent compound, in which both singlet excitons and triplet excitons are engaged in emission, has been developed for the OLED.
The red and green phosphorescent compound having a relatively high efficiency are introduced and developed. However, there is no blue phosphorescent compound meeting the requirements in emitting efficiency and reliability.